Because Ukraine is pro-west and Crimea is pro-Russia. West good, Russia bad. Sanctions are black and white. The political situation is less-so:

The Crimea voted in a (illegal) referendum to rejoin Russia after a coup in Ukraine (also illegal, obviously) had replaced the elected but somewhat pro-Russian president with an unelected pro-European government.

While this revolution was popular with the west (funny how we support coups that overthrow elected governments when they go our way), it was unpopular in the south of Ukraine, which is mostly ethically Russian (the previous ethnic population having all been ‘relocated’).